Dogs, sheep, pigs, cows, horses - all these animals and more have been fundamentally changed by humans to make our lives better. Domestication has fundamentally altered the course of human history, reshaping the land and other species to fit our new agricultural lifestyle. But how do you take a wild species and turn them into domesticated helpers? Why can animals like dogs or horses be domesticated while their close relatives like foxes and zebras remain stubbornly wild?

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What is domestication?

Domestication is when humans take a plant or animal species and, through selective breeding, transform the species into something beneficial for humans. Historically, there have been three major reasons why humans domesticate. The first is to create a ready source of food, such as taking the wild ancestors of cows and turning them into cattle. The second is to harness animals for work, which can range from transportation to use in war, with horses being the most obvious example of this. Then there's domestication that creates pets and companions for humans - dogs are a great example of this.

When we think of domestication, we typically think of animals, but plants can also undergo a fairly radical domestication process. Some plant species remain virtually indistinguishable between their wild and domesticated forms, but others have very particular traits selected for and made dominant during domestication, as we'll discuss in a moment. Like animals, domesticated plants can serve a couple of purposes: crops are all the plants we domesticated for food, while house plants are all the ones we chose because they look pretty decorating our homes.

Before we go on, it's crucial to distinguish between domestication and taming. A single wild animal can be tamed if it is captured at a young age and raised with a lot of careful human nurturing. But this is strictly an acquired trait, and a single taming does not suddenly make the entire species domesticated. And, on the other hand, it is possible for a domesticated animal to return to the wild, although then it's a feral species, not a wild one.

When did humans start practicing domestication?

As with many of these deep historical questions, it's tricky to say with any real certainty. As humans in the Middle East began to switch from hunter-gatherer societies to agricultural ones about eleven or ten thousand years ago, they must have begun experimenting with how best to grow crops, and after even just a few generations the crops would have started to show clear signs of selection away from their wild counterparts.

There's evidence of semi-domesticated rye crops in Syria dating back over 13,000 years, although the first fully domesticated plant seems to be the bottle gourd, which had been transformed into a readymade container for human use by about 12,000 years ago. Cereal crops like wheat and peas appeared about 11,000 years ago, also in the Middle East, while the first livestock like sheep and pigs were also first domesticated about 11,000 years ago in various locations throughout Asia.

Still, although agriculture and domestication are now inextricably intertwined, it is possible to have one without the other. Hunter-gatherers may have domesticated at least one species long before they began experimenting with farming, as there's evidence that they kept dogs for hunting and companionship a few thousand years before the advent of agriculture. (It's hard to say with more certainty than that - the domestication date veers wildly between as far back as 30,000 years ago and as recently as 9,000.) And, as we recently discovered, there were also apparently some failed attempts to domesticate foxes. Ah, but why did dogs succeed where foxes failed? To put it another way...

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What makes an animal possible to domesticate?

The world's foremost expert on domestication is arguably UCLA Professor Jared Diamond, who is best known for his popular science books like the awesomely titled Guns, Germs, and Steel. In the book, he lays out six basic criteria that an animal has to meet in order to be possible to domesticate. Let's run through them briefly:

1. The animal needs to be able to eat a lot of different thing and be willing to live off the scraps of humans. If the animals are able to eat stuff humans can't, such as grass, then even better. This also makes strict carnivores somewhat more difficult to domesticate than other animals, as it commits humans to providing a ready food source of other animals for them to eat.

2. The animals need to grow up fast, or at least faster than humans. There's not much point in trying to domesticate extremely long-lived species like elephants or tortoises, as it can take several years before they're even remotely useful, and their long life cycle limits how quickly their numbers can be replenished.

3. The animals must be willing to breed in the close quarters of captivity. Any creature that demands a lot of open territory in order to breed - pandas and antelopes are good examples of this - are terrible domestication candidates.

4. The animals have to be naturally pleasant. An unpredictable or ill-tempered beast is just going to be dangerous to attempt to keep enclosed in a small area. It's possible to meet some animals halfway on this - for instance, the American bison can be kept in huge enclosures on ranches - but that's as close as we can get to full-on domestication for species like that.

5. It isn't just pleasantness - the animals need to be calm as well. Skittish or flighty animals will constantly attempt to escape, and it can be almost impossible to control them even if escape is impossible. This is what seems to have kept foxes from being successfully domesticated, as they're far more skittish than dogs and wolves.

6. The animals need to be willing to recognize humans as their new masters, which means they must have a flexible social hierarchy.

The remarkable thing about all this is that some animals that seem almost indistinguishable from their domesticated counterparts can prove to be incurably wild. The difference between foxes and dogs doesn't seem like very much, domestic pigs seem superficially very similar to the impossibly wild peccaries and warthogs, and zebras can even interbreed with the fully domesticated horses and donkeys...and yet outside a couple isolated cases of taming zebras, the species has proven completely impossible to domesticate.

What happens when you domesticate a species?

As a general rule, domesticated species become smaller and dumber than their wild counterparts. The underlying causes of this are simple enough. Domesticated animals expend little to no energy on gathering food, and while they always have food, it's generally of a lower quality than what they could find in the wild. This means there's no reason for the animals to grow particularly big, although they often become quite fat.

The shrinking brain size of domesticated animals has a couple factors behind it. Again, the animals aren't under any real survival pressure, which means there's no selection for the smarter, fitter animals. It's obviously much easier to be dumb in an enclosed pen than it is in the wild, so stupid animals survive that otherwise wouldn't. It's also more energy-efficient to keep the brain size smaller when there's no real need to develop it any further, so if anything domestication starts to select against the smarter animals.

I'm using some evolutionary terminology here, and I need to be careful with that. Obviously, a few thousand years isn't really enough time for evolutionary processes to occur as they would in the wild, but human intervention has bred towards certain traits and bred out others. This artificial selection has created radically different populations, sometimes even radically different species, and this has altered how evolutionary mechanisms operate on the domesticated species.

Are domesticated species all mutants?

Remember way back in the first section, when I promised I'd discuss in a moment how some plants have otherwise rare traits that are made dominant through domestication? Well, at long last, this is that moment. In particular, I want to consider the example of wheat, a domesticated crop that is almost completely different from its wild counterpart.

The crucial difference is that, when wild wheat releases its seed, they fall to the ground and scatter everywhere. Domesticated wheat, on the other hand, keeps its seed on the stem, which makes replanting much easier - in fact, it might not have even be possible to domesticate wild wheat if this trait had not presented itself. But where did this alternative seeding method originate?

There's some thought that this was actually a chance mutation that occurred just as the first agriculturalists were experimenting with wheat. The farmers seized upon the mutant crops and replanted them exclusively. In time, a rare mutation that might only have affected a tiny fraction of the wild wheat became the dominant form of the domestic crop - all because of a random mutation at exactly the right moment in human history.

This example has led researchers to wonder whether all domesticates are, on one level or another, mutants. Dogs, for instance, might be the descendants of wolves that had mutated to be more comfortable around humans. Obviously, this is a subtle distinction, and the question comes down to whether certain animals always had the basic traits required for domestication, and they just required some selective breeding to bring them out, or whether a chance mutation needs to occur in order to create domestication-ready traits.

That seems somewhat implausible considering just how many different species have been domesticated, and there's some good evidence to suggest selective breeding alone can domesticate even unlikely animals. The best example of this is Soviet scientist Dmitri K. Belyaev's 1950s experiments to domesticate foxes. He repeatedly selected only the foxes that were most comfortable around humans for further breeding, and in just a few decades he had a population of foxes that had no fear of humans...and, intriguingly enough, looked and acted an awful lot like dogs.

Are humans domesticated?

It's an interesting question - have the same domestication processes that we placed upon animals also acted upon humans? Have we domesticated ourselves so that we are fundamentally different from our "wild" ancestors? Well, for the sake of argument, let's go back to Jared Diamond's checklist:

1. Humans will eat just about anything, so we certainly have a flexible diet. Arguably, we eat a greater variety of foods today than we did pre-agriculture, although access to new foods is a key variable there, so we should be careful with that point.

2. Growth rate is kind of hard to assess, as that one is actually defined in terms of growth speed relative to humans. But even here, there's some evidence that our growth rate has accelerated. Agricultural humans experience the onset of puberty much earlier than hunter-gatherers do. Hunter-gatherer females also only experience about a quarter the amount of ovulation cycles than their agriculturalist counterparts, and they breastfeed for far longer. It's subtle, but there's definitely some evidence that we grow up quicker now than we did when we were hunter-gatherers.

3. We're certainly able to breed in some pretty tiny spaces - just ask a college freshman - but there's no real evidence that humans have ever been territorial in their reproductive patterns. Let's mark this one as "Not applicable."

4 & 5. Have humans become more pleasant or calm over time? This is where we get into some seriously shaky territory, but it's worth pointing out that agricultural societies have historically tended to tie humans to one particular enclosed area for the duration of their lives, with their freedom of movement greatly restricted. Contrast this with the nomadic lifestyle of hunter-gatherers, and there's certainly the possibility that humans are more comfortable being "enclosed" than they were in the past.

6. As for our social hierarchy - well, hunter-gatherer societies tend to be egalitarian, with a much reduced emphasis on central leadership. Agriculturalist humans have almost always lived in systems where a particular person or particular group was to be recognized as the clear leader.

Now, none of that can really be considered clinching proof of anything, although it seems fair to say that, in general, the differences between our hunter-gatherer ancestors and modern humans broadly fit onto a path of increased domestication. But then there's our frames and our brains, as we recently saw. Agriculturalist humans have consistently been much smaller than our hunter-gatherer ancestors, and it's only in the last century or so that the average height and size of humans has caught up with the ancient norms. And, just like domesticated animals, our brains are smaller than they used to be - and at least one theory is that we now live in a world where you can get away being stupid.

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That's the thing about domestication - it's an amazing tool that has underpinned so much of human history. It gave our ancestors the food sources necessary to survive living in just one location, animals provided transportation and war machines that would otherwise be impossible in a pre-industrial world, and even little household pets and plants have added to the sum total of human happiness. But it's a one-way street: just like most domesticated animals could never hope to survive a return to the wild, we humans are now completely dependent on the continued success of this 10,000-year model. Humans have changed so many animals to fit our needs, but we've arguably changed ourselves most of all.